1. Field of the Invention
The invention relates to a process for moulding high strength elongated parts from a fibre-resin composite material, and is particularly, but not exclusively, applicable to the manufacture of turbomachine blades, such as fan blades.
2. Summary of the Prior Art
The RTM (resin transfer moulding) process is used, particularly in aeronautics, to produce parts made of an organic resin-fibre composite material with a high strength to mass ratio, and comprises the following basic steps:
a) making a fibre preform to the shape of the finished part; PA1 b) placing the preform in a mould; PA1 c) injecting liquid resin into the mould; and PA1 d) maintaining the resin under pressure while polymerizing the resin by heating. PA1 providing a mould defining a mould cavity having the shape of the part to be produced, said mould including a succession of heating and cooling elements spaced apart between opposite ends of said mould cavity in the longitudinal direction thereof; PA1 making a fibre preform; PA1 placing said fibre preform in said mould cavity; PA1 activating said cooling elements; PA1 injecting liquid resin into said mould cavity and maintaining a supply of said liquid resin under pressure to said mould cavity at one end thereof; PA1 successively de-activating said cooling elements at predetermined time intervals starting from the end of said mould cavity opposite said one end and progressing in turn towards said one end; and PA1 successively activating said heating elements starting from said opposite end of said mould cavity and progressing in turn towards said one end substantially in synchronism with said successive de-activation of said cooling elements; PA1 whereby said heating elements raise the temperature of the resin in said mould cavity to the polymerization temperature of said resin progressively towards said one end of said mould cavity and said cooling elements prevent the liquid resin in advance of the gelling front of the polymerizing resin from polymerizing prematurely. PA1 the resorption of shrink cavities is improved when the front is narrow, and PA1 only a limited amount of material can be gelled at one and the same time, because of the rate of flow of pressurized resin through the preform to replenish the resin at the gelling front is restricted by the viscosity of the resin.
The resins used are very fluid and penetrate satisfactorily between the fibres of the preform, even when injected at a relatively low pressure. During polymerization under the action of heat, the resin changes in turn from the liquid state to a gelled state, and then to a solid state. However, these resins suffer from a 4% to 8% volume reduction during the gelling phase, which leads to the development of shrink cavities, i.e. localized absences of material within the finished part.
When the temperature of the part is homogeneous, the surface of the part becomes rough, as the hotter interior of the part polymerizes first and attracts resin which is still liquid from the surface. However, with more bulky parts the bulkier portions heat up and polymerize more rapidly as a consequence of the fact that the polymerization reaction is exothermic, and the heat thus produced dissipates more slowly. Shrink cavities then tend to form in the colder portions, which polymerize last and hence feed the hotter portions with liquid resin. This phenomenon is particularly inconvenient in the case of turbomachine blades, the aerofoil portion of which has a long thin trailing edge in which the shrink cavities tend to form.
It is known to equip the mould with both heating elements and cooling elements to warm the colder portions and cool the hotter portions during polymerization. However, such systems are very difficult to regulate, and are not very effective when the part has substantial amounts of mass in localized areas. Moreover, they do not solve the surface roughness problems.
It is known, from the publication of the statutory invention registration US-H559 dated 6th Dec., 1988, to carry out moulding in a flexible envelope, the method consisting of creating and shifting a gelling front in the part by pressing the part between two plates, one of which is heated and the other cooled, and subsequently progressively heating the cooled plate to the polymerization temperature of the part to shift the gelling front. However, this process is reserved for thin parts of uniform thickness, and it is intended as a means for facilitating the evacuation of the volatile components evolving from the polymerization, and not for resorbing the shrink cavities. Moreover, it does not allow precise dimensioning in terms of thickness.